The development of atomic resolution scannng tunneling microscopy (STM) and spectroscopy (STS) by Binnig and Rohrer [(G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Appl. Phys. Lett., 40, 178 (1982); Phys. Rev. Lett., 49 57 (1982); and G. Binnig and H. Rohrer, IBM J. Res. Dev., 30 355 (1986)] has opened a new era of surface science. The first STMs were based on the original IBM Zurich tripod design in which three orthogonal piezoelectric rods support and scan the tunneling probe, while sample translation is accomplished by means of an electrostatic "louse". The known systems of vibration isolation are of two primary types, i.e., two levels of spring suspension with associated eddy current damping, [(G. Binnig and H. Rohre, Helv. Phys. Acta., 55, 726 (1982)] or the stacked plate arrangement with Viton spacers and springs separating a half-dozen or so metal plates. [Ch. Gerber, G. Binnig, H. Fuchs, O. Marti, and H. Rohrer, Rev. Sci., 181, 92 (1987)]. More recently, the piezoelectrically driven "louse" has given way to the micrometer driven differential spring [B. Drake, R. Sonnenfeld, J. Schneir, and P. K. Hansma, Surf. Sci., 181, 92 (1987)] and stepper motor gear reduction [Sang-il Park and C. F. Quate, Rev. Sci. Instrum., 58, 2011 (1987)] approaches for coarse sample positioning. These techniques are more reliable and the differential spring assembly is easily incorporated into the overall STM design and works well at low temperatures. [A. P. Fein, J. R. Kirtley, and R. M. Feenstra, Rev. Sci. Instrum., 58, 1806 (1987)].
A major problem with tripod scanners is thermal drift, with millikelvin temperature stability required for low drift imaging. [Sang-il Park and C. F. Quate, Appl. Phys. Lett., 48, 112 (1986)]. This situation has been helped by alternate designs such as the thermally compensated matrix STM of van de Walle et al., [G. F. A. van de Walle, J. W. Geritsen, H. van Kempen, and P. Wyder, Rev. Sci. Instrum., 56, 1573 (1985)] and the bimorph driven metal tripod design of Jericho et al. [B. L. Blackford, D. C. Dahn, and M. H. Jericho, Rev. Sci. Instrum., 58, 1343 (1987)]. The thermally compensated matrix design uses small cubes of piezoelectric material arranged such that lateral and z-direction thermal drift cancel out. Although this design has relatively low thermal drift, it is not low enough for variable temperature work, and the design is complex and difficult to build.
A significant step towards simplifying STM design was the development of the tube scanner STM by Binnig and Smith. [G. Binnig and D. P. E. Smith, Rev. Sci. Instrum., 57, 1688 (1986)]. In this design, a single piezoelectric tube with its outer electrode divided into four equal quadrants, parallel to the tube axis, provides lateral scanning motion by tube bending when voltages are applied to two adjacent outside quadrants, and z-displacement when voltage is applied to the common inner electrode. The tunneling probe is affixed to on of the grounded outer quadrants. Because of its simplicity, small size, and rigidity (with associted high resonance frequencies), the tube scanner has replaced the scanning sections of many older STM designs. Due to the tube's symmetry, a coaxially located tunneling probe would not undergo lateral thermal drift for uniform temperature changes. However, the elimination of thermal drift along the z-direction would require some sort of compensation. An effective compensation scheme is to affix the sample holder to a second, concentric piezoelectric tube which is the same length as the scanning tube. This would be the tube scanner analog of the thermally compensated matrix design of van de Walle et al. [G. F. A. van de Walle, J. W. Gerritsen, H. van Kempen, and P. Wyder, Rev. Sci. Instrum., 56, 1573 (1985)].
Although it is reasonably straightforward to thermally compensate the scanning element(s) in an STM, there can still be considerable thermal drift and vibration sensitivity arising from the sample holder and its associated coarse positioning system. Most STM coarse positioning systems incorporate mechanical elements such as springs, levers, gears, micrometers, and stepper motors. These are coupled directly to the sample holder yet they are typically 10.sup.8 times larger than the tunneling gap width. Consequently, thermal drift and mechanical vibration of these elements directly modulates the tunneling gap.
An interesting design, which eliminates these components is the so called "Johnny Walker" STM [K. Besocke, Surf. Sci., 181, 145 (1987)] in which a tube scanner is located symmetrically at the center of an arrangement of several additional tubes. This STM can be operated inverted with a sample being placed directly on the outer legs or non-inverted, in which the STM "walks" over a surface. Walking motion is accomplished by slowly bending and then rapidly straightening the outer legs, resulting in inertial translation of the entire STM. Inertial translation of a mass using a piezoelectric transducer configuration has been demonstrated by Pohl. [D. W. Pohl, Surf. Sci., 181, 174 (1987); and Rev. Sci. Instrum., 58, 54 (1987)]. Coarse positioning with the Johnny Walker STM is a problem requiring the sample be placed on an incline, and its overall size results in the need for vibration isolation and makes variable temperature operation difficult.